Performance of sustainable mortars containing blast furnace slag and fine concrete waste: an environmental perspective

Abstract

About 9% of the globe’s carbon dioxide (CO₂) emissions are attributed to cement production. The replacement of cement with blast furnace slag and fine concrete waste (FCW) could reduce the CO₂ emissions associated with the construction sector. Consequently, the main objective of the current study is to examine the combined effect of blast furnace slag and FCW on mortar manufacturing. The experimental study was conducted in three stages. In stage I, mortars were prepared using cement-to-sand ratio of 1:3. The cement was substituted with FCW at 0.00, 0.30, 0.60 and 0.75 fractions. From the Stage I results, a mortar mixture prepared with 60% FCW and 40% cement was chosen for the Stage II study. In place of cement, slag was used in fractions of 0.00, 0.05, 0.10, 0.15, 0.20 and 0.25. The strengths of the mortars were significantly enhanced when slag was employed as a replacement for cement. In the final stage of the study, the environmental impact (EI) of producing FCW was evaluated. The results show that the EI values for producing FCW are far below those of cement and slag. For instance, the global warming potentials of producing cement, slag, and FCW were 0.951 kgCO₂eq, 0.0188 kgCO₂eq and 0.0169 kgCO₂eq, respectively. In conclusion, the produced mortars provided the specified strength requirements for masonry works as well as environmental and economic benefits, making them sustainable.

Keywords:

• Fine concrete waste
• sustainable mortars
• slag
• cementitious materials

REVIEWING EDITOR:

• Ian Phillip Jones
• University of Birmingham
• UK

SUBJECTS:

• Testing
• Materials Science
• Civil, Environmental and Geotechnical Engineering

1. Introduction

The employment of supplementary cementitious materials (SCMs) as a partial replacement for cement in mortar production is highly recommendable. From the literature, about 9% of the globe’s carbon dioxide (CO₂) emissions are attributed to cement production (Boden & Andres, 2017; Mehta & Monteiro, 2014). The substitution of cement with SCMs could reduce the CO₂ emissions associated with cement manufacturing. Consequently, several studies involving the utilization of fine concrete waste (FCW) or slag in cement mortars have been conducted.

The effect of FCW on the strength properties of recycled mortars was examined by Xianwei and Zhenyu (2013). The performance of the mortars was reported to decrease as the percentage of the FCW increased. The control mortar’s compressive strength declined by 5.9% when 20% of the cement was replaced with FCW. Also, the flexural strength of the control mortar was decreased from 6.4 N/mm² to 4.3 N/mm² upon 30% substitution of cement with FCW. Amar (2021) examined the impact of FCW on mortar’s performance. The author mentioned that the flexural strength of the control mortar was lowered from 9.5 N/mm² to 7.7 N/mm² when 20% of the cement was replaced with FCW. The mechanical properties of cement mortars containing FCW were investigated by Moon et al. (2005). The compressive and flexural strengths of the control mortar were reduced by 19.8% and 11.4%, respectively, when one-fifth of the cement was replaced with FCW.

Wu et al. (2021) examined the impact of FCW on recycled mortars. They reported that the compressive strength of the mortar mixture made with 100% cement and 80% cement + 20% FCW were 42.3 N/mm² and 31.5 N/mm², respectively. Kim and Choi (2012) studied the properties of mortars made with cement and FCW. The control mortar’s compressive strength was reduced by 54.1% upon 30% replacement of the cement with FCW. Similarly, Thays et al. (2020) reported that the mortar mixture prepared with 100% cement was reduced from 49.0 N/mm² to 30.0 N/mm² when 30% of the cement was substituted with FCW.

The percentage reductions in compressive strength and flexural strength of 10% and 20% FCWs mortars experienced by various authors are presented in Figures 1 and 2, respectively. Evidently, the percentage decrease in both properties went up as the FCW content rose. The average percentage reduction in the compressive strength and flexural strength of the 10% FCW mortars increased from 5.5% and 6.1% to 15.6% and 14.2% for the 20% FCW mortars, respectively.

Figure 1. Percentage reductions in 28-day compressive and flexural strengths of 10% FCW mortars.

Figure 2. Percentage reductions in 28-day compressive and flexural strengths of 20% FCW mortars.

Thakur et al. (2016) assessed the effect of slag on the mechanical properties of mortars. Mortars were prepared by replacing cement with slag at 0%, 10%, 20%, 30%, 40%, 50%, 60% and 70%. It was reported that the compressive strength of the mortars increased for the replacement level of slag up to 50%. The 28-day compressive strength of the control mortar increased from 45.3 N/mm² to 47.7 N/mm² for a mortar mixture made with 60% cement and 40% slag. In an investigation by Oleiwi (2021), mortars were produced by substituting cement with slag at 0%, 20%, 30% and 40% contents. It was mentioned that the 28-day compressive strength of the control mortar increased from 35.0 N/mm² to 44.0 N/mm² when 30% of the cement was substituted with slag, representing an increment of 33.3%.

Awang and Aljoumaily (2017) conducted a study to examine the influence of slag on mortars. Mortars were prepared by replacing cement with slag at 0% and 50%. Compressive strength values of 53.1 N/mm² and 54.7 N/mm² were obtained for mortar mixtures prepared with 100% cement and 50% cement + 50% slag, respectively. Alakara et al. (2022) prepared geopolymer mortars by substituting FCW with slag at 60%, 70%, 80%, 90% and 100%. It was mentioned that the strength properties of the mortars increased as the replacement content of slag increased. Mortar mixtures made with 100% slag and 60% slag + 40% FCW experienced 28-day compressive strength values of 53.4 N/mm² and 59.6 N/mm², respectively.

Ngo et al. (2022) prepared sustainable mortars by replacing cement with slag at various ratios. The 28-day compressive strength of the control mortar was increased by 11.3% when 15% of the cement was substituted with slag. Likewise, Sambowo et al. (2021) reported that the 7-day compressive strength of the control mortar was increased by 58.7% when 20% of slag was added to it.

The above information shows that experimental studies on cement mortars containing FCW and slag are available in the literature. However, studies on the combined effect of FCW and slag on cement masonry mortars are yet to be found. Also, studies on the environmental impact (EI) of producing FCW are unavailable in the literature. Therefore, the present study seeks to fill these gaps in the literature.

1.1. Purpose and process of the study

The purpose of the present study was to evaluate the combined effect of FCW and slag on mortar production. Figure 3 outlines the research process. The materials required for the mortar production were collected and prepared. Testing and characterization of the materials were done to ascertain their suitability. Mortar mixtures were designed. The mixing, casting and curing of the mortar specimens were carried out. The properties of the mortars were determined using the appropriate standards. The EI of the mortar’s elements was evaluated. Finally, the obtained results were discussed, and conclusions were drawn.

Figure 3. Flowchart of the research process.

2. Materials and methods

2.1. Materials

The mortars were made using FCW, slag, river fine aggregate and water. The fine aggregate’s grade was consistent with (ASTM C144, 2003). Water employed for the mortar’s preparation conformed to BS EN 1008 (2002). The waste concrete used for the study was secured from a construction site in Gauteng Province, South Africa. The waste concrete was broken into smaller pieces using a hammer. The fine particles of the waste concrete were obtained by using a sieve size of 0.30 mm. The collected fine particles were further ground using a ball mill and sifted through a 0.075 mm sieve. The obtained powders, which are referred to as FCW in this study, were put in plastic bags and sealed until the time for the preparation of the mortars. The physical properties of the mortar elements are presented in Table 1.

Table 1. Physical properties of the mortar elements.

Type of test	Fine aggregate	FCW	Slag
Silt and clay content (%)	3.05	–	–
Bulk density (kg/m^3)	1678.50	879	1100
Moisture content (%)	2.29	–	–
Fineness modulus	2.78
Specific gravity	2.77	2.65	2.93
Blaine fineness (cm^2/g)	–	2818.20	4000.00

2.2. Methods

2.2.1. Analytical techniques

Microscopic examination was conducted using TESCAN VEGA3 scanning electron microscopy (SEM), while laser diffraction particle size analyser Malvern-Mastersizer 2000 Hydro-G was employed to conduct the particle size distribution (PSD) analysis. The oxide composition of the raw materials was determined using a Malvern panalytical spectrophotometer.

2.2.2. Mortars mixtures, preparation and testing

Tables 2 and 3 show the various mortar mixtures in Stages 1 and 2, respectively. In Stage 1, the FCW was used to replace cement in the following proportions: 0.00, 0.30, 0.60 and 0.75. For the Stage 2 mortars, the cement was replaced with slag at 0.00, 0.05, 0.10, 0.15, 0.20 and 0.25 fractions. Steel moulds with dimensions of 50 mm × 50 mm × 50 mm, 40 mm × 40 mm × 160 mm and 100 mm × 200 mm cylinders were used for casting the mortar specimens for compressive strength, flexural strength and splitting tensile strength tests, respectively. Three (3) specimens were cast for each test. The mortar specimens were removed from the moulds 24 h after casting and cured in water. The specimens were tested on the 3rd, 7th, 28th and 90th days of curing. Table 4 presents the tests conducted and their corresponding standards. Also, Figure 4 shows the compressive strength, flexural strength and water absorption tests conducted.

Figure 4. Testing of mortar properties.

Table 2. Mixtures of mortars containing cement and FCW.

Specimen type	Mortar elements (g)
	Cement	FCW	Fine aggregate	Water
Mix-1	586.0	0.0	1758	351.6
Mix-2	410.2	175.8	1758	351.6
Mix-3	234.4	351.6	1758	351.6
Mix-4	146.5	439.5	1758	351.6

Table 3. Mixtures of mortars containing FCW, cement and slag.

Specimen type	Mortar elements (g)
	FCW	Cement	Slag	Fine aggregate	Water
Mix-A	351.6	234.4	0.0	1758	351.6
Mix-B	351.6	222.7	11.7	1758	351.6
Mix-C	351.6	211.0	23.4	1758	351.6
Mix-D	351.6	199.2	35.2	1758	351.6
Mix-E	351.6	187.5	46.9	1758	351.6
Mix-F	351.6	175.8	58.6	1758	351.6

Table 4. Standards for testing the mortar specimens.

Test	Standard
Flow	ASTM C1437 (2007)
Setting time	ASTM C191 (2019)
Apparent porosity	BS EN 1015-10 (1999)
Dry bulk density	BS EN 1015-10 (1999)
Water absorption	ASTM C1403 (2000)
Density	BS 1881-114 (1983)
Compressive strength	ASTM C109 (2013)
Flexural strength	BS EN 1015-11 (1999)
Splitting tensile strength (St)	St = 2 F/πdl

3. Results and discussion

3.1. Material characterization

3.1.1. Oxide compositions of the materials

The oxide compositions of the materials are presented in Table 5. The oxide results of the cement are consistent with those reported by Florea et al. (2014), Kim and Choi (2012), Oksri-Nelfia et al. (2016) and Prošek et al. (2019). From the table, the FCW results are in agreement with those found in the literature (Gastaldi et al., 2015; Martinez et al., 2016; Prošek et al., 2019). In the case of slag, it can be seen that Al₂O₃, CaO, MgO and SiO₂ account for 94.1% of the total oxide.

Table 5. Oxide composition of the materials.

Oxide	Cement	FCW	Slag
Al2O3	5.47	9.05	15.26
BaO	0.00	0.00	0.23
CaO	63.49	23.95	33.35
Fe2O3	2.98	6.89	0.82
K2O	0.41	0.91	0.95
MgO	1.08	3.21	7.62
MnO	0.12	0.24	1.07
Na2O	0.06	1.55	0.24
P2O5	0.07	0.19	0.00
SiO2	21.05	33.75	37.86
SO3	1.44	1.28	1.91
TiO2	0.64	1.01	0.68
LOI	1.93	17.44	0.00

3.1.2. Microstructure and particle size distribution of the materials

The microanalysis of the materials is shown in Figure 5(a–c). Clearly, the cement particles have angular shapes and smooth surfaces. The FCW particle has less angular shape compared to that of the cement. Also, the slag particles consist of several irregular shapes with smooth surfaces. Figure 6 presents the PSD of the materials used to prepare the mortars.

Figure 5. (a) Particle morphology of the cement obtained by SEM. (b). Particle morphology of the FCW obtained by SEM. (c). Particle morphology of the slag obtained by SEM.

Figure 6. Particle size distribution of the materials.

3.1.3. Microanalysis of the specimens in Stage 2

Figure 7 shows the microstructure of the mortar specimens prepared in Stage 2. Clearly, the spaces in Mix-A specimen decreased as the slag level increased up to 15%. However, when the substitution ratio of slag surpassed 15%, the pores of the mortar specimens began to increase when compared to that of Mix-A. Using an appropriate replacement level of slag enhanced the bonding ability of the mortars, while high-volume replacement of slag adversely affected the mortar bonding.

Figure 7. Microstructure of the mortar specimens prepared in Stage 2: (a) Mix-A, (b) Mix-B, (c) Mix-C, (d) Mix-D, (e) Mix-E and (f) Mix-F.

3.2. Influence of FCW on strength performance in Stage 1

The compressive strength values of the mortars are shown in Figure 8. Apparently, the incorporation of FCW into Mix-1 led to a reduction in compressive strength, regardless of the curing day. The 28-day compressive strength of the mortar mixtures Mix-1, Mix-2, Mix-3 and Mix-4 were 39.7 N/mm², 34.6 N/mm², 26.8 N/mm² and 20.1 N/mm², respectively. This indicates that the compressive strength of Mix-1 declined by 32.5% when 60% of the cement was substituted with FCW. The reduction in compressive strength could be ascribed to the inactive elements found in FCW, which affected the hydration process of the mortars (Kim & Choi, 2012). The result is consistent with other studies found in the literature (Moon et al., 2005; Ohemeng et al., 2023).

Figure 8. Influence of FCW on the compressive strength of the mortars.

3.3. Properties of the mortars in Stage 2

3.3.1. Influence of slag on flow

Figure 9 presents the flow of the mortars. The inclusion of slag in Mix-A led to an increase in the flow values. The flow results of the mortar mixtures Mix-A, Mix-D and Mix-F were 110 mm, 115 mm and 124 mm, respectively. This means that the flow of Mix-A was increased by 12.7% when 25% of the cement was replaced with slag. The increase in flow could be due to the smooth and hard surface of the slag particle, which reduces its absorption capacity during initial mixing (Liu et al., 2022).

Figure 9. Influence of slag on the flow of the mortars.

3.3.2. Influence of slag on setting time

Figure 10 depicts the effect of slag on the mortars’ setting times. The replacement of the cement with slag led to an increase in setting time of the mortars. For instance, the final setting time of Mix-A increased from 565 mm to 578 mm for Mix-D, representing an increment of 2.3%. Slag has latent hydraulicity, which must be activated. When activators such as alkalis react with slag, the glass structure is disrupted, resulting in its activation (ACI 233R, 2003; Ohemeng, 2023). The alkalis in cement activate the slag particles for hydration when cement is replaced with slag. This chemical reaction causes the early hydration of the mortars containing slag to be slowed, thereby retarding the setting time.

Figure 10. Influence of slag on setting time of the mortars.

3.3.3. Influence of slag on density

The effect of slag on the density of the mortars is shown in Figure 11. Generally, the influence of slag on the density of the mortars was insignificant. The 28-day density of the mortar mixtures Mix-A, Mix-B and Mix-C were 2157.6 kg/m³, 2147.7 kg/m³ and 2148.8 kg/m³, respectively. Nevertheless, the density of Mix-A began to decline when the substitution level of the slag exceeded 15%. This could be ascribed to the variations in specific gravity values of the two materials (Table 1).

Figure 11. Influence of slag on the density of the mortars.

3.3.4. Influence of slag on strength properties

The effect of slag on the strength properties of the mortars is shown in Table 6. Evidently, the employment of the slag had significant impact on the mechanical properties of the mortars, regardless of the curing day. For instance, the 28-day compressive strength of Mix-A was increased from 15.8 N/mm² to 16.9 N/mm², 18.3 N/mm² and 16.7 N/mm² for mixtures Mix-B, Mix-C and Mix-D, respectively. This indicates that the compressive strength of Mix-A was improved when the cement content was replaced with slag up to 15%. However, the compressive strength of Mix-A reduced when the content of slag surpassed 15%. Using an appropriate replacement ratio of slag enhances mortar strength, but a high-volume addition of slag leads to a significant reduction in hydration products and strengths (Prošek et al., 2019). Similar trend of results was observed for the other strength properties. For instance, flexural strength values of 4.2 N/mm², 4.5 N/mm² and 4.7 N/mm² were recorded for Mix-A, Mix-B and Mix-C, respectively, at curing day 28. The produced mortars have the potential to be used for masonry mortar types M, S, N and O, which require 28-day compressive strength values of 17.2 N/mm², 12.4 N/mm², 5.2 N/mm² and 2.4 N/mm², respectively (ASTM C270, 2014; Ohemeng & Ekolu, 2019; Ohemeng & Naghizadeh, 2023).

Table 6. Strength properties of the mortars.

Specimen type	Compressive strength (N/mm^2)				Flexural strength (N/mm^2)				Splitting tensile strength (N/mm^2)
	3 days	7 days	28 days	90 days	3 days	7 days	28 days	90 days	3 days	7 days	28 days	90 days
Mix-A	7.70	10.58	15.75	19.30	1.90	3.30	4.22	5.16	0.45	0.92	1.66	2.19
Mix-B	6.95	10.25	16.94	19.48	1.85	3.20	4.45	5.18	0.43	0.87	1.69	2.24
Mix-C	6.50	10.05	18.25	22.35	1.78	3.12	4.69	5.63	0.41	0.83	1.91	2.56
Mix-D	6.21	9.45	16.65	19.89	1.69	3.05	4.36	5.22	0.38	0.80	1.68	2.22
Mix-E	5.87	9.12	14.17	16.58	1.41	2.98	4.01	4.75	0.35	0.78	1.49	1.99
Mix-F	5.32	8.92	13.57	15.92	1.17	2.81	3.75	4.32	0.32	0.74	1.39	1.86

3.3.5. Significant effect of slag on strength properties of the mortars at 28 days of curing

Table 7 shows how much slag explains the variations in compressive strength of the mortars. A coefficient of determination (R²) = 0.775 was obtained, which indicates a strong correlation between the compressive strength and slag content. The table also exhibits the conventional statistical report comprising an adjusted R² = 0.745, F(2, 15) = 23.786, p < .001. Thus 74.5% of the variation in compressive strength of the mortars could be explained by the slag content. It is evident from the table that the effect of slag content on the compressive strength of the mortars is statistically significant, since p < .05. Based on the coefficients given in B-column of Table 7, the model for predicting the compressive strength of the mortars is given in Equation (1)(1) $$C{s}_{28}=0.312x-0.017x^2+15.940$$(1) . (1) $$C{s}_{28}=0.312x-0.017x^2+15.940$$(1) where Cs₂₈ is the 28-day compressive strength and x is the slag content.

Table 7. Regression coefficients of compressive strength of the mortars at 28-day curing.

Model	Unstandardized coefficients		Standardized coefficients	t	Sig.
	B	SE	β
Slag content	.312	.086	1.585	3.625	.002
Slag content 2**	−.017	.003	−2.281	−5.218	.000
Constant	15.940	.457		34.886	.000

R² = 0.775 (adjusted R² = 0.745); F(2, 15) = 25.786, p < .001.

** represent exponent or power.

Table 8 illustrates the extent to which slag accounts for the variation in the mortar’s flexural strength. The results show a good correlation between the flexural strength and slag content, with a coefficient of determination (R²) = 0.701. The standard statistical report with an adjusted R² = 0.661, F(2, 15) = 17.582, p < .001, is also displayed in the table. This suggests that the slag content accounts for 66.1% of the change in the mortars’ flexural strength. The table shows that the influence of slag content on the mortars’ flexural strength is statistically significant, since p < .05. Equation (2)(2) $$F{s}_{28}=0.063x-0.003x^2+4.246$$(2) provides the model for determining the mortar’s flexural strength based on the coefficients provided in the B-column of Table 8. (2) $$F{s}_{28}=0.063x-0.003x^2+4.246$$(2) where Fs₂₈ is the 28-day flexural strength and x is the slag content.

Table 8. Regression coefficients of flexural strength of the mortars at 28-day curing.

Model	Unstandardized coefficients		Standardized coefficients	t	Sig.
	B	SE	β
Slag content	0.063	0.020	1.555	3.088	0.007
Slag content 2**	−0.003	0.001	−2.207	−4.384	0.001
Constant	4.246	0.109		39.132	0.000

R² = 0.701 (adjusted R² = 0.661); F(2, 15) = 17.582, p < .001.

** represent exponent or power.

The degree to which slag content explains the changes in the splitting tensile strengths of the mortars is shown in Table 9. The coefficient of determination (R²) = 0.797 indicates a strong correlation between the splitting tensile strength and slag content. Additionally shown in the table is the conventional statistical report with an adjusted R² = 0770, F(2, 15) = 29.429, p < .001. This indicates that 77.0% of the variation in the splitting strength of the mortars may be attributed to the slag content. p < .05 suggests that the slag content has a statistically significant effect on the splitting tensile strength of the mortars. Equation (3)(3) $$S{t}_{28}=0.028x-0.002x^2+1.656$$(3) gives a model for estimating the splitting tensile strength of the mortars using the coefficients provided in the B-column of the table. (3) $$S{t}_{28}=0.028x-0.002x^2+1.656$$(3) where St₂₈ is the 28-day splitting tensile strength and x is the slag content.

Table 9. Regression coefficients of splitting tensile strength of the mortars at 28-day curing.

Model	Unstandardized coefficients		Standardized coefficients	t	Sig.
	B	SE	β
Slag content	0.028	0.008	1.471	3.544	0.003
Slag content 2**	−0.002	0.000	−2.204	−5.310	0.000
Constant	1.656	0.043		38.832	0.000

R² = 0.797 (adjusted R² = 0.770); F(2, 15) = 29.429, p < .001.

** represent exponent or power.

3.3.6. Influence of slag on dry density and apparent porosity

The dry bulk density and apparent porosity results of the mortars are shown in Figure 12. Mortar mixtures Mix-A, Mix-B, Mix-C and Mix-D recorded dry bulk density values of 1906.01 kg/m³, 1910.53 kg/m³, 1914.61 kg/m³ and 1908.55 kg/m³, respectively. Also, the apparent porosity of the mortar mixtures Mix-A, Mix-B, Mix-C and Mix-D were 19.6%, 19.2%, 18.8% and 19.5%, respectively. Clearly, the incorporation of slag up to 15% in Mix-A had no adverse effect on its porosity and bulk density. However, lower bulk densities and higher apparent porosity values were experienced when the substitution level of slag surpassed 15%.

Figure 12. Influence of slag on dry bulk density and apparent porosity of the mortars.

3.3.7. Influence of slag on water absorption

The effect of slag content on water absorption of the mortars is shown in Figure 13. Evidently, the amount of slag incorporated in Mix-A has a significant impact on its water absorption rate. The water absorption of Mix-A declined for slag content up to 15%. For 4-h immersion, water absorption values of 37.43 g/100 cm², 36.63 g/100 cm², 35.76 g/100 cm² and 36.92 g/100 cm² were recorded for Mix-A, Mix-B, Mix-C and Mix-D, respectively. This indicates that the water absorption of Mix-A was decreased by 4.5% when 10% of the cement was substituted with slag. However, as the replacement content of slag exceeded 15%, higher water absorption values were recorded when compared to the control mortar. The trend of the results is supported by the microstructure analysis of the mortar specimens (Figure 7). Figure 13 further shows that the specimens experienced rapid water absorption rate beyond 4 h of immersion (approximately 61% of the total water absorption).

Figure 13. Water absorption of the mortar mixtures at different durations.

4. Environmental impact of producing fine concrete waste

The EI analysis of FCW was done from the angle of commercial production. Three (3) stages were considered in the analysis. In the first stage, the EI of transporting waste concrete from demolition site to recycling plant located in the Gauteng Province of South Africa was calculated. Average distance of 58 km and transportation type (a truck of Euro 2 class) were used. For the second stage, the EI for processing the FCW at the recycling plant was considered. A dataset for average use of diesel fuel machine operations was used to model the work of the equipment at the recycling plant. The total energy of 2.258 kWh/ton was considered based on the collected information about the capacities and working regimes of the machines. The distribution among processes is shown in Table 10. In the third stage, the EI for sieving the 75 µm FCW was modelled by the consumption of electrical energy. Also, the EI for the calcination of the FCW at 500 °C for 4 h was determined at this stage. The average electricity mix for South Africa from the Ecoinvent database was used. The EI indicators were determined in accordance with (EN 15804, 2012). The Ecoinvent database version 3.6 and the openLCA software were used to model the processes. All calculations were done for 1 tonne of material. The different environmental elements considered were: abiotic depletion potential (ADP), global warming potential (GWP), ozone depletion potential (ODP), photochemical ozone creation potential (POCP), acidification potential (AP) and eutrophication potential (EP). Figure 14 and Table 11 show the results for the various environmental elements.

Figure 14. Environmental impact of processing fine concrete waste.

Table 10. Relative share of energy used for processing 1 tonne of the material.

Process	%
Sorting process	3.54
Primary crushing	33.22
Conveyor	0.89
Magnetic separation	19.54
Secondary crushing	33.22
Washing or air-sitting	6.64
Screening	2.95

Table 11. Environmental impact for producing fine concrete waste without calcination (1 tonne of material).

Process				ADP			EP
	GWP-fossil (kg CO2 eq)	ODP (kg CFC11 eq)	POCP (kgC2H4eq)	Elements (kg Sb-Eq)	Fossil (MJ)	AP (kgSO2eq)	Freshwater (kgPO4eq)	Marine (kgNeq)	Terrestrial (molcNeq)
Transport of waste concrete to recycling plant	15.74	0.000	0.043	0.000	250.533	0.053	0.004	9.04E − 03	0.094
Recycling plant processing: sorting, crushing, separation, washing, screening	0.75	1.61E − 07	0.002	1.14E − 06	10.14	1.28E − 03	2.71E − 05	1.65E − 04	1.83E − 03
Sieving of FCW	0.42	4.36E − 09	0.002	4.83E − 07	8.10	5.67E − 03	3.00E − 04	7.50E − 04	7.76E − 03
Total	16.91	1.65E − 07	0.047	1.62E − 06	268.77	0.060	4.33E − 03	9.96E − 03	0.104

4.1. Comparing the environmental impact of producing the various binders

The EI values for producing 1 kg of FCW determined from the current study were compared to those of cement and slag found in the literature (Blengini, 2006; Chen et al., 2010). Figure 15 shows the EI of the cementitious materials employed in this study. Obviously, cement has the highest EI values, followed by slag and FCW. For instance, the GWPs of producing cement, slag, and FCW were 0.951 kgCO₂eq, 0.0188 kgCO₂eq and 0.0169 kgCO₂eq, respectively. The partial replacement of cement with slag and FCW for the manufacturing of mortars leads to a reduction in the EI values of the produced mortars, making them eco-friendly.

Figure 15. Environmental impact values of producing 1 kg of the various binders.

5. Conclusions

The present study evaluated the effect of FCW and slag on cement mortars. The FCW and slag were used to partly replace cement in the mortar production. Also, the EI of producing FCW was assessed in this study. The following major findings are given:

1. The strength properties of the mortars were enhanced when slag up to 15% was employed. For instance, the 28-day compressive strength of the control mortar was increased from 15.8 N/mm² to 18.3 N/mm² when 10% of the cement was replaced with slag.

2. The produced mortars met the criteria for masonry mortar types M, S, N and O, which have a minimum 28-day compressive strength of 17.2 N/mm², 12.4 N/mm², 5.2 N/mm² and 2.4 N/mm², respectively. Also, the mortar mixture containing 60% FCW, 30% cement and 10% slag was identified as the optimum proportion.

3. Based on the obtained results, it is recommended that the replacement of cement with slag in mortars containing high volume of FCW should not exceed 15%.

4. The study has shown that the EIs of producing FCW are far below those of slag and cement. For example, the ADP of producing cement, slag and FCW were 0.00399 kgSbeq, 0.00032 kgSbeq and 1.62 × 10⁻⁹ kgSbeq, respectively.

5. The incorporation of slag and FCW in mortars makes them eco-friendly since the EIs of producing slag and FCW are lower than those of cement.

Author contributions

Eric A. Ohemeng: conceptualization, data curation, formal analysis, investigation, methodology, visualization, writing – original draft preparation, and editing. Molusiwa S. Ramabodu: conceptualization, review, editing, and supervision. Tholang D. Nena: conceptualization, review, and editing. Yana Kancheva: conceptualization, review, and editing. All authors read and approved the final manuscript.

Acknowledgements

This article is part of the doctoral study of Eric A. Ohemeng. The authors appreciate the contribution and support of the technicians at the laboratory of the Civil Engineering Science Department, University of Johannesburg.

Disclosure statement

The authors declare that we have no potential conflict of interest.

Data availability statement

The datasets used and/or analysed during the current study are available from the corresponding authors upon reasonable request.

Additional information

Funding

The authors received no direct funding for this research.